Open AccessResearch Numerical simulation of in vivo intraosseous torsional failure of a hollow-screw oral implant Murat Cehreli*1, Murat Akkocaoglu2 and Kivanc Akca3 Address: 1 Associat
Trang 1Open Access
Research
Numerical simulation of in vivo intraosseous torsional failure of a
hollow-screw oral implant
Murat Cehreli*1, Murat Akkocaoglu2 and Kivanc Akca3
Address: 1 Associate Professor of Prosthodontics, CosmORAL Oral and Dental Health Polyclinics, Cinnah 7/5 Kavaklıdere, Ankara, Turkey,
2 Associate Professor, Department of Oral Surgery, Faculty of Dentistry, Hacettepe University, 06100 Sihhiye, Ankara, Turkey and 3 Associate
Professor, Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, 06100 Sıhhiye, Ankara, Turkey
Email: Murat Cehreli* - mccehreli@yahoo.com; Murat Akkocaoglu - makkocao@hacettepe.edu.tr; Kivanc Akca - akcak@hacettepe.edu.tr
* Corresponding author
Abstract
Background: Owing to the complexity and magnitude of functional forces transferred to the
bone-implant interface, the mechanical strength of the interface is of great importance The
purpose of this study was to determine the intraosseous torsional shear strength of an
osseointegrated oral implant using 3-D finite element (FE) stress analysis implemented by in vivo
failure torque data of an implant
Methods: A Ø 3.5 mm × 12 mm ITI® hollow screw dental implant in a patient was subjected to
torque failure test using a custom-made strain-gauged manual torque wrench connected to a data
acquisition system The 3-D FE model of the implant and peri-implant circumstances was
constructed The in vivo strain data was converted to torque units (N.cm) to involve in loading
definition of FE analysis Upon processing of the FE analysis, the shear stress of peri-implant bone
was evaluated to assume torsional shear stress strength of the bone-implant interface
Results: The in vivo torque failure test yielded 5952 μstrains at custom-made manual torque
wrench level and conversion of the strain data resulted in 750 N.cm FE revealed that highest shear
stress value in the trabecular bone, 121 MPa, was located at the first intimate contact with implant
Trabecular bone in contact with external surface of hollow implant body participated shear stress
distribution, but not the bone resting inside of the hollow
Conclusion: The torsional strength of hollow-screw implants is basically provided by the marginal
bone and the hollow part has negligible effect on interfacial shear strength
Background
Following the introduction of osseointegrated oral
implants to rehabilate functional and esthetic
conse-quences related to the loss of teeth and associated hard
and soft tissues, a variety of criteria have been placed to
evaluate short- and long-term implant success [1-3]
Despite the efforts to optimize implant healing and
main-tenance of bone-implant interface, early and late implant
failures are still reported At present, commonly cited fac-tors leading to implant failure are biological and biome-chanical, but the initiation of marginal bone loss remains essentially unclear
Marginal bone loss to a certain level, particularly within the first year of function, is accepted as a physiologic reac-tion Nevertheless, peri-implantitis and functional- or
Published: 04 November 2006
Head & Face Medicine 2006, 2:36 doi:10.1186/1746-160X-2-36
Received: 15 May 2006 Accepted: 04 November 2006 This article is available from: http://www.head-face-med.com/content/2/1/36
© 2006 Cehreli et al; licensee BioMed Central Ltd
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Trang 2over-loading seem to be role-mates in progressive bone
loss beyond the clinically-accepted limits, and likely result
in failure of the bone-implant interface Although various
treatment modalities [4-7] have been described to control
(micro)damage of peri-implant tissues, the biological
competence of the bone-implant interface is
questiona-ble, particularly under fatigue-induced mechanical
fail-ures, where the interface stiffness plays a critical role
Due to the prerequisite of direct bone implant contact per
se coined as osseointegration [8], a great deal of scientific
endeavors are constantly being focused on the
biome-chanics of bone-implant interface for long-term success of
implant-supported prostheses Interactions between bone
and implants can be explicitly analyzed and even
quanti-fied through histologic and histomorphometrical
proce-dures, yet these techniques can not be used as the only
criterion for characterization of the implant-bone
inter-face In fact, the "mechanical" competence of biological
ankylosis needs to be clarified with regard to complex oral
forces acting indirectly on bone-implant interface The
bone-implant interface is commonly tested via pushout,
pullout and torque mechanical experiments to quantify
the established shear strength, but currently available data
are limited to either various animal studies [9] or in vivo
experiments of temporary implants [10] However, the
lack of consistency between animal models and geometric
implant designs seriously questions the consistency with
real-time biological data Moreover, in vivo mechanical
experimental tests of smaller diameter temporary
implants with machined surface do not represent the
actual bone-implant interface strength In order to
improve current knowledge on the mechanical properties
of the interface, the purpose of this biomechanical study
was to quantify failure torque of an osseointegrated
implant with severe bone loss and involve the in vivo data
in finite element (FE) analysis to define torsional shear
strength at yield
Materials and methods
Clinical findings and torque failure test
A 62 year-old male patient applied for treatment of
exten-sive breakdown of implant and teeth supported fixed
prostheses that have been in functioning for 8 years both
in maxillary and mandibular partially edentulous arches
In the maxilla, one-piece acrylic veneered fixed prosthesis
was present on teeth # 13 and # 27, and implants were
placed at #14, # 21, # 23 and # 26 In the mandible, the
roots of the teeth # 35 and # 47 were present, and a Ø 3.5
mm × 12 mm ITI® hollow screw dental implant (Institut
Straumann, Waldenburg, Switzerland) was present in
place of tooth # 46 without any restoration (Fig 1)
Detailed dental history revealed that, the mandibular
fixed prosthesis recently droped-off spontaneously with
two implants According to treatment planning based on
clinical and radiographical examinations, and diagnostic prosthetic set-up, explantation of the mandibular implant, yet not mobile was suggested Prior to implant removal, the procedure was explained to the patient and a written consent was obtained Referring to rough and smooth implant surface border, the mean (mesial and dis-tal) vertical and horizontal bone loss measured on digi-tized periapical radiograph using a software for image analysis (ImageJ 1.34n, NIH, USA) were 7.55 mm and 4.15 mm, respectively (Fig 2) Biological parameters and assessed mean values at four aspects of the implant during clinical examination were as follows; modified plaque index (MPI)11: 3, modified bleeding index (MBI) [11]: 3, the distance between the implant shoulder and the mucosal margin (DIM): 1.94 mm and the peri-implant probing depth (PPD): 8.25 mm No suppuration was observed around the peri-implant soft tissue
Torque failure of the existing bone-implant interface was tested using a custom-made strain gauged manual torque wrench [12] During application of screwing torque force via couple of surgical (Institut Straumann) and rachet adapter (Institut Straumann), the strain-gauge signals were recorded by a data acquisition system (ESAM Travel-ler 1, Vishay Micromeasurements Group, Raleigh NC, U.S.A) and corresponding software (ESAM; ESA Messtech-nik GmbH, Olching, Germany) at a sample rate of 10 KHz (Fig 3) Due to lack of interlocking feature between implant and any compatible article in product range (Institut Straumann), the torque could not be applied in counter-clock direction to remove the implant In essence,
it was beyond the scope of the study to remove the implant, but to quantify the failure torque of the implant
in bone The force applied to the handle of manual torque wrench was transferred as "torque" to the bone-implant interface along the implant axis via the surgical- and rachet-adapter Therefore, the quantification of applied torque to the bone-implant interface was essential to implement this information to finite element analysis for quantification of intraosseous failure torque of the test implant In this regard, the strain data of the manual torque wrench was converted to torque units (N.cm) according to the procedures explained elsewhere [13,14]
In brief, the strain data were converted to torque units (N.cm) using the general formula:
Torque = K x ε
where K is the calibration constant and ε is the
strain-gauge reading Then, torque failure output was imple-mented in definition of loading conditions of the
simula-tion of in vivo experimental circumstances using finite
element stress analysis
Trang 3Finite element stress analysis
The in vivo experimental circumstances were simulated
using finite element stress analysis to get more
informa-tion about the biomechanical properties of the
bone-implant interface In this regard, 2-D model of the Ø 3.5
mm × 12 mm ITI® hollow screw dental implant (Institut
Straumann) and the solid abutment (Institut Straumann)
were constructed in one-piece using pre-processor,
MSC.Marc Metat 2005 (MSC Software Corporation, Los
Angeles, CA) Helical continuity in threads of the implant
body was not considered in modelling, but as
symmetri-cal rings [15] The implant-abutment model was centrally
and vertically positioned into a Ø 20 mm cylindrical
trabecular bone representative with angular peri-implant
bone defect of vertical and horizontal bone loss of 7.5
mm and 4 mm respectively 3-D finite element (FE)
model conversion was performed by 3600 axial rotating of
planar model using MSC.Marc Metat 2005 (MSC
Soft-ware Corporation) A fully-bonded interface was defined
for the implant body in the bone simulant Eight-node
isoparametric hexahedral elements were used in 3-D FE
model conversion and resulted in 27.300 and 31.500
ele-ments in implant-abutment and bone, respectively (Fig
4a) The calculated torque unit (N.cm) from on the strain
data obtained during the clinical test that yielded failure
of the bone-implant interface was implemented in defini-tion of loading condidefini-tions in the finite element analysis
In the definition of loading condition, a centrally located
node (# 17827) on the occlusal surface of the abutment
was selected and retained All other nodes resting on the occlusal surface and their degree of freedom (dof) were connected to centrally retained node using RBE 2 link (MSC.Software Corporation) Then the rotational torque force was applied onto the centrally-retained node along the implant axis (Fig 4b) Rotational torque force that yield to failure of bone-implant interface was applied on
the occlusal surface of the solid abutment to simulate in vivo load application Boundary conditions were
estab-lished by constraining the cylindrical bone circumferen-tially and from its bottom The FE analysis solver, MSC.Marc 2005 (MSC.Software Corporation), was used for processing the rotational torque force application All materials were assumed to be homogenous, isotropic and linearly elastic with Young's modulus and Poisson's ratio for implant-abutment complex 110,000 MPa and 0.35, respectively, and trabecular bone 1850 MPa and 0.3, respectively In addition, no further definition was consid-ered to define bone-implant contact due to lack of vali-dated data concerning absolute shear bond strength of bone-implant interface Scalar results of shear stress in
Panoramic view of both jaws and the implant subjected to torsional failure test in the right premolar region
Figure 1
Panoramic view of both jaws and the implant subjected to torsional failure test in the right premolar region
Trang 4trabecular bone representative were evaluated using
post-processor, MSC.MarcMetat 2005 (MSC Software
Corpo-ration)
Results
During in vivo torque failure test of bone-implant
inter-face, implant spinning was not evident, but at the
moment of failure, bleeding from the peri-implant sulcus
and partial loss of torque resistance of the implant was
observed The in vivo torque failure test yielded 5952
μstrains, as determined from the computer software
Con-version of in vivo strain data to torque units revealed that
the torque failure of the bone-implant interface occurred
at 750 N.cm
As a sequel of finite element analysis, high shear stress
val-ues were recorded circumferentially at the first intimate
contact of trabecular bone with implant surface Torsional shear stresses at first contact with trabecular bone and consecutive two thread tips in descending order to implant apical, and were 121 MPa, 109 MPa, and 97 MPa, respectively (Fig 5a and 5b) Trabecular bone in contact along with external surface of hollow implant body, except the bottom regions of threads, experienced lower shear stress values ranging between 72 – 24 MPa, and dis-tribution of stresses through trabecular bone were limited
to 250 μm (Fig 5b) Shear stresses at the trabecular bone interface resting in hollow section of implant body were ranging between 12 – 0 MPa (Fig 5b) Overall, the stress distribution in failure test revealed that the highest stresses were recorded in the occlusal aspect, lower stresses
in the implant body, and very low stresses within the hol-low part of the implant where, intimate bone contact was present
The periapical radiograph of the hollow-screw implant with extensive marginal bone loss
Figure 2
The periapical radiograph of the hollow-screw implant with extensive marginal bone loss
Trang 5Following intraosseous placement, achievement and
maintenance of direct bone-implant contact is of utmost
importance for optimum long-term functioning of oral
implants One of the major concerns regarding
mechani-cal integration of implants is the interfacial strength between the bone and the implant Therefore, evaluation
of shear strength of bone-implant interface with pushout and pullout experiments are required to test mechanical competence of orthopedic and oral implants In essence, the rationale behind the common use of uniaxial testing
is the relative simplicity in the experimental procedures [16] If the shear strength of bone-implant interface is being tested, outcomes of torque failure tests are more dependable, when moment forces in oral function are
considered In this regard, ex vivo torque failure studies to
test bone-implant interface are abundant [17-19] In addi-tion, nominal shear strength of bone-implant interface also has been calculated mathematically in some studies [20-22] Owing to different experimental circumstances including test sites, species and material configuration, the consistency of these techniques with actual clinical conditions are questionable Because of ethical considera-tions, available human torque failure data are either lim-ited to a study [10] carried on transitional implants or a case report [23] of 2 non-loaded conventional implants Unlike previous studies, in the present biomechanical study, shear stress state of bone-implant interface was evaluated using FE analysis As creating a consistency between models and biological data is the main objectives
in biomechanical studies, the applied load definition was based on clinical torque failure test of the simulated implant and peri-implant conditions Biological
condi-tions and mechanical test procedure might affect the in vivo data Advanced peri-implantitis place an argument
The manual torque wrench with adapter connected to
implant
Figure 3
The manual torque wrench with adapter connected to
implant
a) The finite element model of the implant Note that, approximately 30–35% bone loss is present around the implant, although the hollow part is totally filled with bone b) The centrally-retained nod and the nodes attached to this node is presented in red color The rotational force is applied at this node, which coincides with the implant- or the y-axis (purple)
Figure 4
a) The finite element model of the implant Note that, approximately 30–35% bone loss is present around the implant, although the hollow part is totally filled with bone b) The centrally-retained nod and the nodes attached to this node is presented in red color The rotational force is applied at this node, which coincides with the implant- or the y-axis (purple)
Trang 6regarding validity of osseointegration In addition to lack
of peri-implant radiolucency, acute infection with
suppu-ration and mobility was not associated clinically for the
implant tested Therefore, the clinical/radiologic status of
the implant, as suggested within a recent consensus report
[24], rendered the existence of osseointegration for
accu-rate torque failure measurement In the present study, the
torsional load was applied in clock-wise direction for the
measurement of interfacial bond failure Perception of
"start to debonding" was referred to initial torque failure
of bone-implant interface during experiment In other
words, peak torque output that likely yielded complete
loosening of implant in bone was not considered in this
study, because the validity of the output would have been
speculative due to probable apical bone resistance to
screwing of the implant In the present finite element
analysis, a linear solution was performed, the contact
between the implant-abutment interface, and the
implant-bone interface, namely, contact analysis, was not
undertaken During clinic test, because the force was
applied in the clock-wise direction and abutment
loosen-ing did not occur, a linear solution did not influence the
outcome of the study Taking the limited bone support of
the implant into account, it would be very useful to
"define" the "contact" in detail between bone and the
implant and the properties of bonding, if possible In
essence, the "core" this study was based on this rationale,
as there is no information dealing with the magnitude and
nature of contact bond between an implant with bone so
far In the present study, the authors assume that there has
not been any limitation of quantification of failure torque
in the clinic test, but the implementation of this
informa-tion to a finite element model with a defined "bond" at the contact surfaces could be very useful The information obtained in the present study could, therefore, be used in future studies to define "bond" in contact analysis of bone-implant interface
In the present study, evaluation of the shear stress state of peri-implant bone revealed that trabecular bone within hollow part of the implant body did not contribute to interfacial shear strength This finding is very important clinically, as the one of the rationale behind fabricating such hollow-screw implants was to increase bone-implant contact and improve the biomechanical performance of these implants The very low magnitude stresses within the hollow part, in comparison with the higher stresses in the outer aspect demonstrate that it is the surface of the implant, particularly the marginal bone region that bears the failure load Indeed, highest shear stress, which likely indicates the location of "start to debonding", was observed at the first intimate contact of trabecular with the implant surface This, in part, may also explain why time-dependent bone resorption takes in the marginal bone region, although higher loads and stresses occur in the apical part of loaded implants It is also very interesting to note that the screw threads resist torsional load to a great extent, as low magnitude stresses were observed on the implant body between the threads This also implies that the design of threads, particularly at the collar region of implants is crucial [25], should decrease peak interfacial shear stresses, and provide optimum distribution of stresses in order to decrease the risk of microdamage in bone during clinical loading Because a very high strain
Peak interfacial shear stresses around the implant demonstrating high shear stresses at the junction of bone implant contact and very low stresses within the hollow part
Figure 5
Peak interfacial shear stresses around the implant demonstrating high shear stresses at the junction of bone implant contact and very low stresses within the hollow part
Trang 7Publish with BioMed Central and every scientist can read your work free of charge
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gradient was needed to fail the implant having
approxi-mately 30–35% bone contact, it is tempting to speculate
that an osseointegrated implant may present more that
three-fold increase in torsional strength than achieved in
the present study (121 MPa) Yet, further studies are
required to substantiate our claims
Competing interests
The author(s) declare that they have no competing
inter-ests
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